In rubber articles such as tires, belts, hoses, and the like, which need to be reinforced with any of reinforcements such as steel cords, organic fibers, and the like, strong adhesion between the rubber and the reinforcement is demanded. For providing adhesion between the rubber and the reinforcement, a method in which the reinforcement is treated with any of various adhesives, and a method in which an adhesive is blended together with various other compounding agents in a processing step (kneading step) of rubber, are known. In particular, the method in which an adhesive is blended in the processing step of rubber is widely used, since this method achieves strong vulcanization adhesion, regardless of the presence or absence of an adhesive treatment for the reinforcement. As an adhesive used in this processing step of rubber, a co-condensate is known obtained by reacting an alkylphenol such as p-tert-octylphenol, p-nonylphenol, or the like with formalin to produce a co-condensate, and by reacting the co-condensate with resorcin. [For example, Japanese Patent Laying-Open No. 06-234824 (PTD 1)].
In recent years, however, p-tert-octylphenol and p-nonylphenol are among candidate substances of very high concern (SVHCs) defined by the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) Regulation within the EU region, and their use within the EU region is more likely to be restricted from now on.
Under such circumstances, the present inventors substituted these substances with p-tert-butylphenol, which is one of p-substituted phenols not included in the list of candidate SVHCs defined by the REACH Regulation, and is available at low cost, to produce the co-condensate described above. As a result, they ascertained that the co-condensate has a very high softening point. Adhesives used in the processing step of rubber are demanded to soften in the rubber processing step, and in the field of rubber for tires where co-condensates (phenolic resins) are commonly used as adhesives, it is known that the rubber processing step is generally performed at around 170° C. [for example, see the Journal of the Society of Rubber Science and Technology, Japan, Vol. 73 (2000), No. 9, pp. 488-493 (NPD 1)]. Hence, in order to ensure that the co-condensate is softened and dispersed in rubber in the rubber processing step, the softening point of the co-condensate is required to be sufficiently lower than a maximum temperature during the rubber processing, that is, 150° C. or lower.